In general, a transmission of a vehicle drive train, such as a vehicle drive train of a commercial vehicle or a mobile drive unit, is intended to mechanically convert power from the crankshaft of the engine in a different speed and torque level, and to transmit this to the drive wheels of an output drive. Transmission devices of commercial vehicles often consist of two serially arranged transmissions. One of the gears, which is also referred to as a splitter, thereby carries out small transmission jumps, while the other gear, which is referred to as a group or as a main gearbox, is intended to present large transmission jumps. Such combinations allow the transmission to carry a high number of transmission stages.
The gear constructed as both a splitter and as a group can be an automatic powershift transmission. During changes of transmission ratios in two serially arranged transmission devices comprising a transmission, in principle, two different types of power shifts arise, so-called “simple power shifts” and so-called “group change power shifts”. During simple power shifts, for carrying out a requested change of transmission ratio solely by the splitter, one transmission ratio is changed, while for group change power shifts, a change to the transmission ratio to carry out the requested change of transmission ratio occurs with both the groups and of the splitter transmission.
During a group change power shift, the two power shifts by the splitter transmission and by the group transmission run simultaneously, and must be coordinated accordingly, whereas, during the change of transmission ratio from the output transmission ratio to the target transmission ratio within the transmission device, a larger transmission jump must be overcome within the transmission device. This results in a large change in the rotation speed of a shaft connecting the group transmission and the splitter transmission with one another, which is also referred to as the central shaft. Under certain circumstances, the large change in rotation speed of the central shaft that occurs during the group change power shift leads to a significant influence of a rotation speed at the output drive and, as a result, to an increased component load, and may also impair drive comfort to an undesirable extent. These disadvantages occur particularly in very high transmission jumps, or even a large amount of inertia in the central shaft.
With the method known from conventional practice, a requested change of transmission ratio in transmission devices is used depending on fixed pressure characteristics and/or characteristics of torques, which are in turn selected depending on the particular operating situation of a vehicle drive train, among other things depending on the current type of shifting, a current drive torque of a drive unit and the like, and depending on which the shifting elements involved in the change of transmission ratio, such as clutches or brakes, are correspondingly activated. The pressure characteristics and the characteristics of each torque to be adjusted at the shifting elements are varied until, starting from a current operating state of a vehicle drive train and thus a transmission device, a desired rotation speed characteristic is adjusted in the transmission device. The application of these processes is effected depending on a multiple of operating parameters, and is therefore very elaborate.
One approach that simplifies the application is the use of a mechanical transmission model for determining the pressure characteristics and/or each torque to be adjusted at the shifting elements. In determining the pressure or torque characteristics, two basic approaches must be distinguished. With one approach, the pressure or torque characteristics are calculated directly through the mechanical transmission model for the shifting elements involved in carrying out a change of transmission ratio, whereas simplifications in the model must be undertaken for this. In the course of development, it has turned out that the simplifications are not acceptable in some cases. For such cases, special functions with their own application parameters are provided, but these are characterized by a high degree of complexity, and lead to higher application effort.
With another approach, determining the pressure and torque characteristics takes place offline with the assistance of optimization approaches. Such an optimization is known from the 2004 dissertation attributed to Wolfgang Klos with the title “Group Shifting Control of Commercial Vehicle Transmissions,” of the University of Stuttgart, reports of the Institute for Engine Construction and Transmission Construction, Prof. H. Binz, report no. 508.
Characteristics of control pressures, on the basis of which the shifting elements involved in carrying out a requested shifting and/or a requested change of transmission ratio of a transmission device, are prescribed as ramp functions, whereas each of the slopes of the ramps form the optimization parameters. An optimization takes place only for the shifting elements and/or clutches to be switched on or closed in the power flow of a vehicle drive train, while the course of a shifting element to be opened during the change of transmission ratio is not affected in the optimization problem. Furthermore, the target quantities underlying the optimization are defined through the quadratic sum of the deviations of the simulated torque and rotation speed characteristics of desired optimal characteristics, whereas a relatively complex model of the vehicle drive train is used. The optimization of the complex model requires an elaborate global optimization method. A so-called “grid search” is used as an optimization method, which can only be used for a few optimization parameters. With this approach, an online optimization by control unit is not feasible, since, on the one hand based on the elaborate optimization method, and on the other hand through the underlying complex model built component-by-component, the optimization is not feasible within the operating times required for this.